The composition of the continental river weathering flux deduced from seawater Mg isotopes

This study establishes the magnesium isotopic composition of seawater and evaluates its constancy as a function of depth and geographic location. It also provides results on the magnesium isotopic composition of river water samples draining specific lithologies. In combination, the results are used to obtain an average magnesium isotopic composition of +1.54‰ for the global continental weathering flux, compared to the +2‰ value of the global ocean (both relative to NIST SRM 980). Analyses of modern and fossil carbonate-secreting echinoderms suggest that these can serve as archives of seawater δ26Mg values. It is demonstrated that these records of the magnesium isotopic composition of ancient oceans can be used to make important inferences about the relative contribution of different lithologies to the global continental weathering flux, particularly carbonate versus silicate weathering. Preliminary results suggest that the composition of the continental weathering flux has not undergone dramatic changes during the Phanerozoic.